Field of the Invention
The present invention is directed to a method for shimming (i.e., reducing inhomogeneities in) the static magnetic field in a nuclear magnetic tomography system, as well as to an apparatus for achieving such shimming.
Description of the Prior Art
In nuclear magnetic resonance devices, the homogeneity of the basic magnetic field is a decisive factor for the imaging quality. During imaging, inhomogeneities in the field in the image area cause geometrical distortions of the field that are proportional to the field deviations. Field homogeneity is particularly important in echoplanar processes. Furthermore, for NMR spectroscopy extremely high demands are made on field homogeneity in order to achieve a sufficient resolution of the spectral lines. Field Inhomogeneities lead to overlapping of the spectral lines.
In order to achieve the required homogeneity levels, a so-called passive shimming is first carried out, i.e. iron plates (for example) that improve the homogeneity are attached in the magnet. An arrangement of this sort is known e.g. from U.S. Pat. No. 5,235,284. This passive shimming, however, is insufficient at high precision levels. For this purpose, special shim coils are additionally provided, charged with an adjustable current. Linear field deviations, i.e., first-order field errors, can also be compensated by supplying gradient coils with an adjustable offset current, i.e. a constant current on which is superimposed a gradient pulse sequence. This is known e.g., from U.S. Pat. No. 5,345,178.
Adjustment of the currents for the shim coils and adjustment of the offset currents for the gradient coils has to be carried out under high-precision conditions while the subject being examined is located in the magnetic field, since under some circumstances the presence of the subject influences the distribution of the magnetic field. A precondition for the adjustment of the shim currents is first of all the precise knowledge of the existing field distribution.
In P. Webb and A. Macovski, "Rapid, fully automated, arbitrary volume, in vivo shimming," SMRM Abstracts 1990, p. 541, a method is disclosed in which two 3-D gradient echo sequences with different echo times are first carried out. On the basis of these two sequences, phase images are generated and then are subtracted. The magnetic field distribution can be three-dimensionally acquired, and M data points can be obtained that are represented in the form of a vector x. Furthermore, a "reference map" is formed in which the influence of M shim coils on the magnetic field is represented in the form of a matrix A. The shim currents to be determined are represented in the form of a vector C. The required shim currents are finally calculated by minimizing the quadratic deviation of the quantity A.multidot.C-x.
From U.S. Pat. No. 5,345,178, a shim method is known in which, for the purpose of calculating the magnetic field inhomogeneity, a gradient echo sequence or a spin echo sequence with non-central 180-degree high-frequency pulses is first generated. The nuclear magnetic resonance signal thus obtained is Fourier-transformed and the phase curve of the nuclear spins in a predefined region is calculated. This is iterated for different projections. The thus-obtained phase curves are analyzed using a curve fit method, and the coefficients of the spherical harmonic functions that describe the field distribution are determined therefrom. The current to be fed to the individual shim coils is thereby determined.
In this type of determination of the field distribution, however, the following problem arises. In the examination of biological tissue with the usual proton imaging, signal portions from the protons bound in fat appear alongside the dominant signal portions from water molecules. These protons have a resonant frequency that is easy to distinguish in comparison with the water protons, due to their different chemical bond. In a magnetic field of strength 1.5 T, the frequency deviation between fat protons and water protons, for example, is around 110 Hz. If one now wished to distinguish reliably the frequency shift caused by the frequency difference in the fat and water signals from the frequency shift caused by magnetic field inhomogeneities, it would already be the case, before even beginning the active shim procedure, that the frequency shift caused by the inhomogeneities could not be greater than 110 Hz over the entire volume under consideration. This is, however, an extreme demand that can hardly be met in practice.